Attempts have been made in the prior art to remove and inhibit the acid mist arising over the tops of such plating tanks. In order to understand this aspect of the problem, a brief description of the electrowinning process for the reduction of copper interior of an electrolytic tank will be set forth. In the description of the process, the need to maintain ready access to the electrodes of the tank will be understood. Thereafter, a summary of the attempted solutions of the prior art will be set forth--together with their known shortcomings.
Modern electrowinning occurs in corrosion resistant tanks--typically made of plastic or plastic fiber concrete mixtures. These tanks are relatively large; they can be about 6 meters long, 1.2 meters across, and 1.4 meters deep, containing in the order of 8 cubic meters of electrolyte containing copper sulfate dissolved in a sulfuric acid solution.
Each tank is provided with an array of depending typically flat electrodes. The electrodes are alternating planar cathode and anode electrodes suspended from the top of the tank and depending downward into the depth of the tank to a depth less than the total depth of the tank. The anodes are provided somewhere along their length with anode insulators; these insulators prevent direct anode to cathode shorting and maintain minimum anode/cathode spacing sufficient for the desired plating. Typically the cathodes, onto which the metal is plated, are larger than the anodes and provided with edge strips. These edge strips cause plating to occur only on the sides of the cathodes so that the copper when plated can conveniently be removed from the flat planar cathode surface. Provision is made for the inflow of fresh electrolyte at one tank end and the outflow of depleted electrolyte at the opposite tank end.
Naturally, the electrodes are communicated with sufficient electrical current to cause the electroplating to occur. Consequently, bus connections to each tank combine to form electrical connections to each electrode resulting in the current between the electrodes to produce the required plating.
In the typical electrowinning process, the anodes are in large measure left in place. The cathodes must be periodically removed for the harvesting of the plated copper. Typically, the tanks are maintained as a group under a common roof in an otherwise large building referred to in the industry as a tank house. This imposes two practical requirements upon the tanks.
First, ready overhead access for the removal and insertion of the cathodes must be available. Second, the electrical connections--which are in a naturally corrosive environment--must be maintained in a relatively conductive state.
Having described the electrowinning environment this far, and remembering that the primary problem is the prevention of the escape of the acid mist, caused by the oxygen gas escaping during the plating process, the prior art attempts to alleviate this problem can now be set forth.
It has been realized in the industry that conventional covering of such tanks is not satisfactory. First, such covering interferes with the required ready access for the cells; removing and replacing a cover before cathode removal or other tank service is not desirable. Secondly, the covering of the electrical connections to the anodes and cathodes is not desirable. Corrosion and depositions under covers destroys conductivity and builds resistance. Finally, acid mist coalesces on the covers in a concentrated format. It then drips down onto the covered electrode supporting parts and connections of the tank, causing corrosion and shorting. As a consequence, for at least these reasons, such covers are not used.
The most commonly used expedient is voluminous ventilation. Massive amounts of air are circulated through such tank houses in the hopes that the acid mist can be swept away before its corrosive effect can harm the health of workers or the interior of the building and its contents. Unfortunately, this is not satisfactory. Worker health may be impaired. Further, the interior of such buildings is an environment in which corrosion rapidly occurs. Attempting to solve this kind of pollution with atmospheric dilution is not satisfactory.
Layers of plastic balls or other acid-inert particles have also been attempted. The theory behind these floating layers is to form a circuitous path for the aerosol from the bursting bubbles--and thereby to attenuate the emission of mist to the environment. This does result in some mist reduction. The emitted aerosol to a limited extent condenses out on the floating objects and finds it way back to the bath. Unfortunately, acid mist or aerosol is still emitted in significant quantities. Therefore, while this expedient is commonly utilized, it does not constitute a complete solution to the problem.
An additional attempt to mitigate this problem has involved utilizing surfactant in the upper layers of the sulfuric acid bath. The theory is that the reduced surface tension of the acid solution will retard the incidence of bubble formation. While this works only to a limited extent, it has a severe drawback.
It will be remembered that the electrolytic solution is circulated through the bath on a continuous basis. When the solution leaves the bath, it goes through a solvent extraction process which enriches the copper content of the solution so that it can be returned to the tank for further electrolysis. This solvent extraction process is a precise, two phase chemical process in which contaminating surfactant should be avoided. Simply stated, no matter how elaborate the precautions taken, sooner or later surfactants find their way into the solvent extraction process--and the process must be halted. Solution must be replaced, and production is lost. Given that the placement of surfactants only results in a partial abatement of the problem, surfactant because of their interference with the solvent extraction side of the process are seldom used.
Other attempts at solution of this problem have likewise been made. In Smith et al. U.S. Pat. No. 4,668,353 issued May 26, 1987 entitled METHOD AND APPARATUS FOR ACID MIST REDUCTION, coalescing of aerosol is taught by providing surface limiting electrically inert masking device in which one portion is submerged in the electrolyte. The idea behind the device is to locally coalesce the mist and redeposit the coalesced acid back into the bath. Emission of aerosol still results.
In an alternate solution, partial "roofing" of the bath was attempted utilizing spanning eaves attached to the anode spanning to the cathode. Two effects occurred. First, the aerosol mist still escaped. Secondly, and during the reinsertion of the cathodes, sulfuric acid dripped from the underside of the eaves onto the harvested and freshly cleaned stainless steel cathodes. These cathodes, representing a significant investment of the total electrowinning process, were etched--especially where they extended above the bath. This being the case, this attempt was abandoned.
In short, a solution has not thus far been found for the vexing problem of the aerosol or mist of acid in electrowinning or electroplating processes.